1. Field of the Invention
The present invention relates to the bonding of metals, the bonding of ceramics or the bonding of ceramics to metals and the like, and in particular to a bonding apparatus suitable for the bonding of parts of precision machines.
2. Description of the Related Art
It is known that the pressure welding of, for example, metals is possible by moving spaced apart surfaces to be pressure welded toward each other, at which an inter-atomic force of gravity can act.
However, since a surface coat such as an oxidized coat or a nitrified coat usually exists on a metal surface, it is difficult to pressure weld metals to each other.
Thus, a method, in which surfaces to be pressure welded are held at high temperatures and the diffusion bonding of materials having a surface coat that does not have strong chemical bonds therein is carried out by utilizing the diffusion phenomenon of said surfaces to be pressure welded, has been proposed as a general pressure welding method. This method is defective in that since the materials to be pressure welded are heated to temperatures near the recrystallization temperatures thereof, metallographical change and thermal strain are produced in the portion bonded and as a result, it is unsuitable for the bonding of parts of precision machines requiring high accuracy.
On the other hand, a method has been proposed in which the surface coat layer is removed by friction between the surfaces to be pressure welded or by brushing the surfaces to be pressure welded with a metal brush instead of heat treating the surfaces and thus, pressure welding at high temperatures is rapidly carried out before the surface coat has formed again.
However, this method is also defective in that since a large strain and a great amount or heat are induced on the surfaces to be pressure welded, the dimensional change of the portions to be pressure welded is inevitable.
As described above, pressure welding is remarkably dependent upon the surface state of metals, so that it is almost impossible to perform in the event that the surface coat layer exists. Accordingly, pressure welding is possible if a clean surface, on which the surface coat layer does not exist, can be obtained. It is, however, remarkably difficult to remove a surface coat without imparting any strain to the surface, and it is difficult to remove the surface coat layer while imparting only a very remarkably small surface strain.
One of the present inventors found from his various investigations that metals can be readily bounded to themselves without any metallographical change or increase in thermal strain of the material pressure welded, or without any dimensional change of the portion pressure welded, or without requiring any special means such as means for heat treating the metals, by pressure welding them in a superhigh vacuum of 10.sup.-9 mmHg or more, after removing a metal surface coat by spattering using inert gas ions (Japanese Patent Application No. 53-32416).
However, since a bonding chamber must be returned to an almost atmospheric pressure condition when the substances to be bonded are put in or removed from the bonding chamber, it takes considerable amount of time to return the pressure inside of the bonding chamber to 10.sup.-9 mmHg or more when the substances are to be bonded. Accordingly, an improvement in productivity cannot be expected.